As shown in FIG. 1, PET equipment is known in which positrons emitted from a positron emission nuclide 8 by the decay of +β undergo pair annihilation with surrounding electrons, and thus generated annihilation radiations 8a, 8b at 511 keV are determined by a pair of radiation detectors 10a, 10b according to the principle of coincidence. In this case, since only the annihilation radiations to which energy at 511 keV has been imparted are utilized, energy (signal) to be obtained is restricted for a lower limit and an upper limit by an energy window. Thereby, the position at which the nuclide 8 is present can be localized on one line segment connecting between the pair of detectors 10a, 10b (coincidence line: line-of-response: LOR). When an axis from the head of a body under testing to the feet is defined as a body axis, a distribution of the nuclide on a planar surface intersecting perpendicularly with the body axis is obtained by image reconfiguration in two-dimensional mode from data of the coincidence line determined on the planar surface in various directions.
A PET detector 10 is a collection of micro-detecting elements, the cross section of which is approximately 5 mm by 5 mm, and requires approximately 2 cm to 3 cm in thickness in order to detect at high probability a pair of annihilation radiations 8a, 8b oppositely emitted from the body. Further, in order to capture the pair of annihilation radiations, the detectors 10 are in general arranged in a ring shape so as to cover a subject, as shown in FIG. 2. However, radiation which is made incident obliquely into the detector 10 will cause a measurement error so as to deteriorate spatial resolution, for which there is no choice but to make the diameter of a ring much larger than the visual field.
In PET equipment, in order to acquire a higher detectability, a three-dimensional detector has been developed for detecting a depth position as well at which the radiation is made incident into a detecting element. As exemplified in FIG. 3, detecting elements of the same type 21 to 24 are stacked on a light receiving element 26, and an optical reflector placed between the detecting elements is used to control the path of light, thus making it possible to localize a depth detecting position and energy from a difference in signals output from the light receiving element 26 (refer to Japanese Published Unexamined Patent Application No. 2004-279057 (Patent Document 1), H. Murayama, H. Ishibashi, H. Uchida, T. Omura, T. Yamashita, “Design of a depth of interaction detector with a PS-PMT for PET,” IEEE Trans. Nucl. Sci., Vol. 47, No. 3, 1045-1050, 2000 (Non-patent Document 1)). Further, two layers are identified for depth in general by stacking two types of detecting elements for each layer to localize a depth detecting position from a time difference in signals output from the light receiving element 26.
The above-described three-dimensional detector 20 is able to improve the deterioration of spatial resolution resulting from radiation made incident obliquely into the detecting elements. Further, the detector 20 can be brought closer to a body under testing than a detector used in the conventional PET equipment, thereby performing detection at a higher sensitivity.
On the other hand, as a method for improving the sensitivity, there is presented an idea of utilizing detector scattering shown in FIG. 4(A). In a conventional two-dimensional detector 10, as shown in FIGS. 4(B) and (C), the detector scattering cannot be distinguished from scattering from a body under testing (also referred to as a scatterer) 6. Therefore, as shown in FIG. 5 and FIG. 6, a lower limit of an energy window is adjusted to that of energy at photoelectric absorption A, by which both scattering events are eliminated as noises.
It is noted that, as shown in FIG. 7, a shield 12 for removing low-energy scattered radiation is installed on the upper face of the detector 10, thus making it possible to remove scattered radiation from the body under testing 6. However, the shield also removes partially a photoelectric absorption event (refer to G. Muehllehner: “Positron camera with extended counting rate capability,” J. Nucl. Med. Vol. 16, 663-657, 1975 (Non-Patent Document 2)).
Thus, PET equipment on which three-dimensional detectors are mounted is able to adopt an arrangement of the detectors so as to give a higher sensitivity than the conventional PET equipment. Nevertheless, there is a disadvantage that a high sensitivity measurement method and a great amount of information that the PET equipment has in principle are not yet utilized to a full extent.